A new Apple patent  filing suggests the company might want subsidize the price of its computers with a complex advertising program that maintains the attention of the user and can lock the computer if the user spaces out.
According to PatentlyApple , the list of machines eligible for the program would run the entire Apple gamut from iPod to desktop computer. But for this kind of scheme to work, OS X would have to be built with all kinds of enforcement mechanisms to make sure that ads could not be disabled or interrupted. The ads would be targeted based on the user's behavior on the machine: the sites, videos, and music they view would all become fodder for the ad system.
As they explain: "In the case of a desktop or notebook, the UI and its components (e.g., menu bars, icons, etc.) may be faded, darkened, brightened, blurred, distorted or otherwise visually modified during the initial state (or while the advertisement is being presented) so as to emphasize that the desktop UI is temporarily inactive."
Though Apple has had a reputation for robust security, implementing a system like this would encourage serious under-the-hood chicanery for hackers seeking the holy grail: a subsidized but ad-free Mac. Should Apple decide to pursue an ad-supported OS X, it'll have to be an immutable code-bunker, which would mean turning off access to some of the built-in features--access to Terminal, for example--that once set it apart from Windows.